1. Field of the Invention
This invention relates to electrical heaters comprising conductive polymers.
2. Introduction to the Invention
Electrical heaters of many different kinds are well known. Some are series heaters, e.g. mineral insulated heating cables, and others are parallel heaters which comprise two (or more) electrodes, e.g. wires or metal foils, and at least one resistive heating element which is connected in parallel between the electrodes. In one important class of parallel heaters, the heating element comprises a conductive polymer composition; preferably at least a part of the conductive polymer composition exhibits PTC (positive temperature coefficient) behavior, i.e. a rapid increase in resistivity at a particular temperature or over a particular temperature range, so that the heater is self-regulating. The term "conductive polymer" is used herein to denote a composition comprising an organic polymer (this term being used to include polysiloxanes) and, distributed therein, a particulate conductive filler. The term "switching temperature" or "T.sub.s " is used herein to denote the temperature at which the rapid increase in resistivity of a PTC composition takes place. When the increase takes place over a temperature range, as is usually the case, T.sub.s is defined as the temperature at which extensions of the substantially straight portions of the plot of the log of the resistivity against temperature (above and below the range) cross. Conductive polymers, and heaters comprising them are disclosed, for example, in U.S. Pat. Nos. 3,861,029, 4,072,848, 4,177,446, 4,242,573, 4,246,468, 4,271,350, 4,272,471, 4,309,596, 4,309,597, 4,334,351, 4,421,582, 4,426,339, 4,429,216, 4,436,986, 4,459,473, 4,520,417, 4,543,774, 4,547,659, and 4,582,983, and in copending, commonly assigned U.S. application Ser. Nos. 720,117 (MP0922-US2), 780,524 (MP0897-US3), 787,218, 818,844 and 864,930 (MP1090-US1, 2 and 3), 818,845 (MP1095-US1) and 818,846 (MP1100-US1). The disclosure of each of the patents and applications listed above is incorporated herein by reference.
A problem which arises with all heaters is that if the heating element or one of the electrodes is broken, or if there is a short between the electrodes, for example as a result of the presence of water (or other conductive liquid), this can cause an arc fault which can have serious consequences, including initiation of a fire. The currents produced in the electrodes by an arcing fault are not necessarily such as to blow the fuse or circuit breaker through which the heater is connected to the power supply.
One use for self-regulating conductive polymer strip heaters is in electric blankets, and U.S. Pat. No. 4,436,986 (Carlson) proposes a safety circuit for such use which is intended to disconnect the heater if a break occurs in one of the electrodes, and thus to prevent ignition of the conductive polymer as a result of arcing at the break. The circuit requires electrical connection to be made at each end of the heater and makes use of a safety circuit which comprises at least one gas tube and which senses the voltage changes produced by an open circuit in one of the electrodes.
It is also known to provide a conductive polymer heater with a grounding plane, e.g. a metal braid around a strip heater or a metal plate on one or both sides of a sheet heater, and to connect the electrodes to a power supply through a ground fault equipment protective device (GFEPD), i.e. a device which constantly compares the current entering the heater in one electrode and the current leaving the heater in the other electrode and which disconnects the heater if the ratio between the currents differs from unity by some preselected amount. In this way, the heater is disconnected if physical damage to it causes one of the electrodes to become connected to ground. However, ground fault equipment protective devices are expensive, and do not operate at all unless the fault involves loss of current to a ground (or, more accurately, to any current sink). Thus they are of no use at all on non-grounded systems, and fail to detect faults, even on grounded systems, unless the arcing fault is accompanied by a ground fault.